In inspection systems used with processing machine installations, such as for semiconductor fabrication ADI and AEI operations, the lamps that are used for illumination during optical inspections are of the tungsten-halogen type, and particularly are called Marco lamps, which typically are operated 24 hours a day irrespective of whether they are needed by the machine operators. This continuous operation is due to the fact that such lamps have a start-up phase that causes the tungsten to vaporize and combine with the halogen atoms but, as the bulb walls may still be too cold to allow the cycle to start, the vaporized material returns to the filament and start-up becomes difficult and power consuming. Consequently, once the lamp is successfully started there is a reluctance on the part of the machine operators to turn it off when not needed and then later to attempt the start up process again when use is required.
Tungsten-halogen lamp characteristics are known and it is clear, as seen in FIG. 1, that if the operating voltage is 10-12% lower than the specified voltage, the lamp life time will be lengthened. The particular tungsten-halogen lamp, the Marco lamp, that is used in ADI and AEI operations is a constant current device and is independent of the operation of the fabrication machines. If, in order to reduce the specified operating voltage and extend the lamp life, a voltage-dropping resistance is included in the lamp circuit, the intensity of the light produced will be lowered, resulting in dissatisfaction on the part of the machine operators since it will become more difficult to identify defects such as photoresist pitting or particle contamination. At present no system exists for achieving an adequate balance between long lamp life and optimum lamp intensity and operation.
It is therefore an object of the present invention to provide a longer life for tungsten-halogen lamps in inspection machines without reducing the operating light intensity of the lamp and degrading the inspection operation.
It is another object of the invention to provide a longer life Marco lamp for use in ADI and AEI operations without reducing the lamp intensity or operating effectiveness during inspections.